Large area solid state light sources have application in display elements and lighting sources. Electroluminescent device such as light-emitting diodes (LED) represents an alternative to conventional display and lighting elements. Electroluminescent devices are opto-electronic devices where light emission is produced in response to an electrical current through the device. The physical model for EL is the radiative recombination of electrons and holes. Both organic and inorganic materials have been used for the fabrication of LEDs. Inorganic materials such as ZnS/Sn, Ga/Bs, Ga/As have been used in, e.g. semiconductor lasers, small area displays, and LED lamps. However, the drawbacks of inorganic materials include difficulties to process and to obtain large surface areas and efficient blue light.
Organic materials offer several advantages over inorganic materials for LEDs, such as simpler manufacturing, low operating voltages, the possibility of producing large area and full-color displays. Conjugated polymers such as poly(phenylvinylene) (PPV) were first introduced as EL materials by Burroughes et al. in 1990 (Burroughes, J. H. Nature 1990, 347, 539-41). Progress has been made since then to improve the stability, efficiency, and durability of polymeric LEDs (Bernius, M. T. et al., Adv. Mater. 2000, 12, 1737). Organic LED (OLED) represents an alternative to the well-established display technologies based on cathode-ray tubes and liquid crystal displays (LCDs), especially for large area displays. OLED has been demonstrated to be brighter, thinner, lighter, and faster than LCDs. Moreover it requires less power to operate, offers higher contrast and wide viewing angle (>165 degree), and has great potential to be cheaper to make, especially the solution processable polymer-based LEDs (PLED). Polymers with wide energy band gap to emit blue light are important materials because stable, efficient blue-light-emitting materials with high brightness are desirable for full color EL display applications. With these primary materials, it is possible to produce other colors by a downhill energy transfer process. For instance, a green or red EL emission can be obtained by doping a blue EL host material with a small amount of green or red luminescent material.
Incorporating an efficient chromophore into a polymer side chain is an effective approach to obtain blue light-emitting polymer. The main chain of the polymer can provide necessary physical properties such as mechanical and film-forming properties and the side chain provides desired electro-optical properties. Blue-light-emitting polymers (Li et al. Syn. Met. 1997, 84, 437) have been prepared by this approach. However, the polymers show high threshold voltages and operating voltages. Thus, it is desirable to develop processable new blue-light-emitting polymers with low driving voltages for full color display.